On Wed, Jan 04, 2017 at 03:12:04PM +0100, Frederic Weisbecker wrote: > When the tick is stopped and an interrupt occurs afterward, we check on > that interrupt exit if the next tick needs to be rescheduled. If it > doesn't need any update, we don't want to do anything. > > In order to check if the tick needs an update, we compare it against the > clockevent device deadline. Now that's a problem because the clockevent > device is at a lower level than the tick itself if it is implemented > on top of hrtimer. > > Every hrtimer share this clockevent device. So comparing the next tick > deadline against the clockevent device deadline is wrong because the > device may be programmed for another hrtimer whose deadline collides > with the tick. As a result we may end up not reprogramming the tick > accidentally. > > In a worst case scenario under full dynticks mode, the tick stops firing > as it is supposed to every 1hz, leaving /proc/stat stalled: > > Task in a full dynticks CPU > ---------------------------- > > * hrtimer A is queued 2 seconds ahead > * the tick is stopped, scheduled 1 second ahead > * tick fires 1 second later > * on tick exit, nohz schedules the tick 1 second ahead but sees > the clockevent device is already programmed to that deadline, > fooled by hrtimer A, the tick isn't rescheduled. > * hrtimer A is cancelled before its deadline > * tick never fires again until an interrupt happens... > > In order to fix this, store the next tick deadline to the tick_sched > local structure and reuse that value later to check whether we need to > reprogram the clock after an interrupt. > > On the other hand, ts->sleep_length still wants to know about the next > clock event and not just the tick, so we want to improve the related > comment to avoid confusion. > > Acked-by: Rik van Riel <[email protected]> > Reviewed-by: Wanpeng Li <[email protected]> > Reported-by: James Hartsock <[email protected]> > Cc: Ingo Molnar <[email protected]> > Cc: Rik van Riel <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
